In modern cellular radio systems, the radio network has a strict control on the behavior of the terminal. Uplink transmission parameters like frequency, timing, and power are regulated via downlink control signaling from the base station to the terminal.
In order for the base station (also called Node B in WCDMA terminology) to control the terminal (also called user equipment, abbreviated to UE in WCDMA terminology), measurements on an uplink signal are required. The determination of timing misalignment requires a comparison of the timing of the received signal with a reference clock in Node B. Timing misalignment is caused by unknown propagation delay and the mutual drift between the clocks in the Node B and in the UE. Likewise, the determination of the transmit power requires a measurement of the received power in Node B in comparison with some threshold. The received power not only depends on the UE transmit power but also on the signal attenuation during propagation. The latter will differ over time.
Uplink measurements are rather straightforward when the UE has established a duplex connection with the Node B. In that case, uplink signals are present for the measurements, whereas downlink signals can carry the control signaling to adjust the UE parameters. However, when the UE is not connected but is in standby, it only listens to the downlink control signal periodically. Thus, there is no uplink signal for the Node B to measure. Before connection establishment, the UE has to carry out a random access (RA) procedure. This is initiated by the UE transmitting a random access burst through a radio interface to the network, which is received by Node B. During the random access procedure, uplink parameters like timing and power are not very accurate. This poses extra challenges to the dimensioning of a random access procedure.
Usually, a physical random access channel (PRACH) is provided for the UE to request access to the network. This means that random access bursts must be detected with good confidence and, when detected, used for propagation delay estimation. The used access burst (AB) contains a preamble with a specific bit sequence that has good auto-correlation properties. The PRACH can be orthogonal to the traffic channels (TCH). For example, in GSM a special PRACH slot is defined. Because multiple UEs can request access at the same time, collisions may occur between requesting UEs. A contention resolution scheme has to be implemented to separate the UE transmissions. This scheme usually includes a random back off procedure. The timing uncertainty is accounted for by extra guard time. The power uncertainty is usually less of a problem as the PRACH is orthogonal to the traffic channels.
In WCDMA, the PRACH is shared with the uplink traffic channels. The uplink channels are not orthogonal. In addition to interference from other requesting UEs, interference is experienced from uplink traffic channels and vice versa. The processing gain provided by the Direct-Sequence spreading will have to cope with the mutual interference. In WCDMA, the transmit power is a shared radio resource. In order to avoid near-far problems, the power received at Node B has to be approximately equal for each UE.
Traffic channels in LTE, i.e. an uplink (UL) basic transmission scheme, are described, e.g., in the document 3GPP TR25.814, v7.1.0 (2006-09), “Physical Layer Aspects for Evolved UTRA”, Technical Report, Technical Specification Group Access Network, 3rd Generation Partnership Project (Rel.7). Here, a cyclic prefix (CP) is inserted in order to enable frequency domain processing. A cyclic prefix (CP) is a sequence of symbols inserted at the start of a block that correspond to a number of symbols at the end of that block. In other words, the cyclic prefix is a repeat of the end of the sequence inserted at the beginning. This means that support for calculating, e.g., Fast Fourier Transforms (FFTs) is needed in an uplink (UL) receiver side of a communication system.
Whenever a discrete Fourier transform (DFT) implemented as an FFT is used on a time domain signal, this time domain signal is assumed to be periodic with period time equal to the length of the DFT. In order to ensure this for signals subject to dispersive channels, a cyclic prefix can be used.